Radiofrequency transmitting/receiving devices require RF clocks (generally of the GHz order) having a high spectral purity (i.e. whereof the component at the central frequency of the spectrum has an amplitude much higher than the other components of the spectrum). The frequency of these clocks must also be easily adjustable, so as to cover the targeted frequency band. Standards generally provide for the allocation of frequency bands, and these frequency bands can be divided into a plurality of consecutive channels. It is therefore necessary to be able to produce a clock whereof the frequency can cover each channel of the frequency band.
Generally, the frequency synthesizers intended to generate these RF clocks are based on the use of a reference clock, with a frequency much lower than the frequency of the RF clock to be obtained. A PLL (Phase-Locked Loop) is then used to multiply the frequency of the reference clock to reach the desired frequency. The use, in the PLL, of a fractional frequency divider makes it possible to obtain an RF clock whereof the frequency is a non-integer multiple of the frequency of the reference clock, thereby allowing a very subtle adjustment of the frequency of the RF clock. The RF clock may be obtained using an LC resonant circuit, and the capacity of which would be variable so as to increase the range of reachable frequencies. The spectral purity of the clock obtained with such a synthesizer can, however, prove insufficient, particularly if the inductance of the LC circuit is made on an integrated circuit, or can then require a very high consumption, which penalizes the autonomy of the system using this type of synthesizer.
Oscillators based on a high-frequency electromechanical resonant element such as a BAW (Bulk Acoustic Wave) make it possible to obtain a spectral purity that is greatly improved relative to the LC oscillator (in the vicinity of 30 dB gain at equal power). A variable capacitive element makes it possible to modify the frequency of the clock produced by such an oscillator, but the variation range of this frequency is not as expansive as that of an LC oscillator with a variable capacity, and generally does not make it possible to cover a wide enough frequency band. Thus, such oscillators are preferably used to generate clock signals at a fixed RF frequency, the aging and dependence of the frequency at the temperature being able to be offset by the use of a more stable reference, such as a quartz.
In order to improve the expanse of the frequency variation range of the clock produced by a BAW, it is possible to divide the frequency obtained by the BAW oscillator, through a fractional division, so as to obtain an IF (Intermediate Frequency) signal, the frequency of which can be varied by modifying the division factor. This IF clock is then mixed with the RF clock coming from the BAW oscillator in order to obtain a variable-frequency RF clock. This solution is not satisfactory, because the fractional division introduces significant phase noise on the IF clock. As a result, after mixing, the phase noise on the variable-frequency RF clock is of the same magnitude as or even greater than that which can be achieved with an LC oscillator. The advantage of the spectral purity of the BAW oscillator is thus lost.
The present invention proposes a frequency synthesizer making it possible to obtain a variable-frequency clock signal with high spectral purity while avoiding the aforementioned drawbacks.